This invention relates to a fuel supply temperature compensation system for an internal combustion engine and, more particularly, to a method for accurately predicting the density of the inlet charge to calculate the correct fuel quantity to be supplied to each cylinder.
In a speed-density fuel injection system it is essential to determine the density of the air entering each cylinder in order to calculate the correct fuel quantity. Since air density is a function of both pressure and temperature, this requires a determination of both the pressure and the temperature of the air entering the cylinder, commonly referred to as the charge pressure and the charge temperature.
It is well known in the automotive industry to measure the pressure using a manifold absolute pressure transducer and to measure the temperature using a sensor mounted in the inlet manifold. However, there is a problem with these temperature sensors in that they typically have a long time constant. For example, a typical temperature sensor may have a time constant of 15 seconds. This long delay results in fueling errors during transient engine conditions. In particular, the measured temperature reads higher than actual on acceleration and lower than actual on deceleration.
In automotive use, due to the lag in the temperature value displayed by these temperature sensors, inaccurate air-fuel ratios result. For example, when a vehicle is decelerating, air flow through the intake passage decreases thereby allowing the air temperature to increase as the air has more time to be heated by the manifold. However, this increased air temperature is not immediately indicated by the temperature sensor, so the resulting air-fuel ratio which is calculated using the inaccurate temperature value is itself inaccurate. Likewise, as the vehicle accelerates air flow through the intake passage increases which results in less time for the air to be heated by the manifold meaning the actual air temperature is lower than the value indicated by the temperature sensor, again causing inaccurate air-fuel ratio calculations.